This invention relates to a method of polycarbonate preparation by solid state polymerization. The method further relates to a method for the preparation of partially crystalline precursor polycarbonates.
Polycarbonates are ranked among the most important of the world""s engineering thermoplastics. Bisphenol A polycarbonate is currently the most widely used polycarbonate and its world wide annual production exceeds one billion pounds. Traditionally, polycarbonates have been prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA)with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide and an amine catalyst such as triethylamine is typical of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.
The interfacial method for making polycarbonate has several inherent disadvantages. First, it is a disadvantage to operate a process which requires the use of phosgene, a reactant the handling, storage and use of which presents important safety concerns. Second, it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to prevent the unintended escape of the organic solvent into the environment. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulates, and higher chloride ion content.
The melt method, although obviating the need for phosgene or an organic solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement of carbonate units along the growing polymer chains. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer""s flow properties and performance.
More recently, polycarbonates have been prepared by solid state polymerization (SSP). SSP offers several advantages over both the melt phase process and the interfacial polycondensation process. SSP does not require the use of phosgene gas which forms an important element of the interfacial process. Additionally SSP utilizes considerably lower temperatures than those required for the preparation of high molecular weight polycarbonate by melt polymerization of a diaryl carbonate such as diphenyl carbonate and a bisphenol such as bisphenol A. Also, the SSP process, unlike the melt phase process, does not require handling highly viscous polymer melt at high temperatures and the special equipment capable of mixing polymer melt under vacuum at high temperature required in the melt process is not required to perform the SSP process.
In a solid state polymerization process, a precursor polycarbonate, typically a relatively low molecular weight oligomeric polycarbonate, is prepared by the melt reaction of a diaryl carbonate such as diphenyl carbonate with a bisphenol such as bisphenol A. In the preparation of bisphenol A polycarbonate oligomers, a diaryl carbonate such as diphenyl carbonate is heated together with bisphenol A in the presence of a catalyst such as sodium hydroxide while removing phenol formed as a by-product of the transesterification reaction between phenolic groups and diphenyl carbonate or phenyl carbonate endgroups. This oligomerization reaction is typically carried out under reduced pressure to facilitate the orderly removal of the phenol by-product. When the desired level of oligomerization has been achieved the reaction is terminated and the product oligomeric polycarbonate is isolated. The oligomeric polycarbonate so produced is amorphous and must be partially crystallized in order to be suitable for solid state polymerization.
The oligomeric polycarbonate may be partially crystallized by one of several methods, such as exposure of powdered or pelletized oligomer to hot solvent vapors, or dissolution of the amorphous oligomer in a solvent such as methylene chloride and thereafter adding a solvent such as methanol or ethyl acetate to precipitate crystalline oligomeric polycarbonate. Typically, such solvent vapor or liquid solvent crystallization methods result in partially crystalline oligomeric polycarbonates having a percent crystallinity between about 20 and about 40 percent as measured by differential scanning calorimetry. A percent crystallinity in this range is usually sufficient for the oligomeric polycarbonate to undergo solid state polymerization without fusion of the pellets or powder being subjected to SSP. In addition to solvent induced crystallization, oligomeric bisphenol A polycarbonate has been crystallized by dissolving diphenyl carbonate in molten amorphous polycarbonate oligomer followed by cooling the mixture to ambient temperature to afford partially crystalline polycarbonate as a mixture with diphenyl carbonate. Finally, amorphous oligomeric polycarbonates have been crystallized by prolonged heating at a temperature below the melting point of the partially crystalline polycarbonate. However, such thermally induced crystallization is quite slow relative to the aforementioned crystallization methods.
The partially crystalline oligomeric polycarbonate in a solid form such as a powder, particulate or pellet is then heated under solid state polymerization conditions at a temperature below the sticking temperature or melting point of the oligomeric polycarbonate, but above the glass transition temperature of the partially crystalline oligomeric polycarbonate, and the volatile by-products formed as chain growth occurs, phenol, diphenyl carbonate and the like, are removed. The polycondensation reaction which converts the low molecular weight oligomer to high polymer is effected in the solid state under these conditions.
Although modern solid state polymerization methods provide a valuable alternative to the melt and interfacial polycarbonate syntheses, the solid state polymerization method suffers from several disadvantages. Typically, the partially crystalline oligomeric polycarbonate polymer precursor must be prepared and crystallized in separate steps, and the solid state polymerization process itself is relatively slow, a typical solid state polymerization step requiring several hours. Thus improvements in the efficiency of the preparation of the partially crystalline precursor polycarbonate and enhancement of solid state polymerization rates are highly desirable.
The present invention provides a method for the preparation of partially crystalline precursor polycarbonates in a single step and their conversion via SSP to high molecular weight polycarbonates. The partially crystalline precursor polycarbonates of the present invention are well suited to solid state polymerization owing to their level of crystallinity and their incorporation of ester-substituted phenoxy endgroups which are more reactive in chain growth reactions with hydroxy endgroups than are unsubstituted phenoxy endgroups. Unsubstituted phenoxy endgroups are present in partially crystalline precursor polycarbonates derived from dihydroxy aromatic compounds and diaryl carbonates lacking ester substitution, such as diphenyl carbonate. These and other objects of the invention will be more readily appreciated when considering the following disclosure and appended claims.
In one aspect, the present invention relates to a method of preparing polycarbonate by solid state polymerization, said method comprising heating to a temperature between about 120xc2x0 C. and about 280xc2x0 C. under solid state polymerization conditions a partially crystalline precursor polycarbonate comprising structural units derived from at least one dihydroxy aromatic compound, and endgroups having structure I 
wherein R1 is a C1-C20 alkyl radical, C4-C20 cycloalkyl radical or C4-C20 aromatic radical,
R2 is independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, C4-C20 aromatic radical, C1-C20 alkoxy radical, C4-C20 cycloalkoxy radical, C4-C20 aryloxy radical, C1-C20 alkylthio radical, C4-C20 cycloalkylthio radical, C4-C20 arylthio radical, C1-C20 alkylsulfinyl radical, C4-C20 cycloalkylsulfinyl radical, C4-C20 arylsulfinyl radical, C1-C20 alkylsulfonyl radical, C4-C20 cycloalkylsulfonyl radical, C4-C20 arylsulfonyl radical, C1-C20 alkoxycarbonyl radical, C4-C20 cycloalkoxycarbonyl radical, C4-C20 aryloxycarbonyl radical, C2-C60 alkylamino radical, C6-C60 cycloalkylamino radical, C5-C60 arylamino radical, C1-C40 alkylaminocarbonyl radical, C4-C40 cycloalkylaminocarbonyl radical, C4-C40 arylaminocarbonyl radical, and C1-C20 acylamino radical; and
b is an integer 0-4.
In another aspect, the present invention relates to the single step preparation of a partially crystalline precursor polycarbonate by the melt reaction of an ester-substituted diaryl carbonate with at least one dihydroxy aromatic compound in the presence of a transesterification catalyst.